DAR altitute is 500' AGL.
If base radar is down, only a dar bar will show in the sector for aircraft above 500' AGL. No discreet dots will show.
The airfield will still blink when any aircraft is inside the radar circle, regardless of radar status.
Radar and how it works in the game is not new, and the reasons for everything have been well though out long ago. As Shane brought up, other means of determing the presense of enemy were available, and the AH system reflects reasonable game play.